tl;dr: Please please please do the conversion first.
JoeTaxpayer's answer is correct, but I am of the opposite opinion.
First, there's just about no reason to have post-tax dollars in a Traditional IRA. You'll eventually have to pay tax on the earnings those dollars generate, so it's essentially the same as having that money in a regular taxable account. Meanwhile, if you roll those dollars into a Roth IRA, you get to earn tax-free money on them for the rest of your life (and even after your death)!
Second, even if you did have some reason for keeping those post-tax dollars where they are, the last thing you ever want to do is mix them with pre-tax dollars (from, say, your 401k). As soon as you mix them, all the dollars become subject to pro-rata taxation (as Joe mentioned), so any future decision you were planning to make about what to do with just your post-tax dollars is moot -- you have given away your right to think separately about your pre- and post-tax dollars.
As an example, let's say the accounts you want to combine look like this:
Current 401k pre-tax : $ 60,000
Current Traditional IRA post-tax: $ 20,000
Hypothetical mixed Traditional : $ 80,000 (75% pre/25% post)
Annoying consequence (no more post-tax-only dollars!)
In the future you decide you want to move $2,000 from the above account into a Roth. Because you mixed the money, the IRS insists that your rollover consists of:
Pre-tax money : $2,000 * 0.75 = $1,500
Post-tax money: $2,000 * 0.25 = $ 500
So now you owe tax (and it's regular income tax, I believe, not even capital gains tax) on $1,500. That was money that you socked away specifically to avoid taxes, and now you've gone and paid taxes on it! Now, there are valid arguments for intentionally moving pre-tax dollars from a Traditional to a Roth like this, but the point is that you shouldn't even have to be having that argument -- you have post-tax dollars in your Traditional IRA that almost certainly belong in a Roth. By mixing your 401k into your Traditional IRA, you can no longer do anything with just the post-tax dollars. The IRS will forever insist that you do these pro-rated calculations.
Really annoying consequence (no back-door Roth for you!)
Say in the future you suddenly realize that a Roth is much better for your financial situation than a Traditional IRA. (Or you might still prefer a Traditional IRA, but as explained in the next sentence it's not available to you.) Unfortunately, because you're covered by a (new) 401k -- or maybe because you earn too much money to contribute pre-tax dollars to either a Traditional or Roth IRA -- you're out of luck. You're simply not allowed to contribute to a Roth.
Most people in this situation can make use of what's called a back-door Roth. They contribute up to the maximum amount per year ($5,500 or whatever it is now) post-tax to a Traditional IRA and then immediately roll it over to their Roth. You can still try this, but guess what? Yep, because you're mixing these new post-tax dollars with pre-tax money in your Traditional IRA, every year your rollover will be tainted with that pre-tax money, diluting the whole point of the back-door Roth. You'll be paying taxes on money you never wanted to pay taxes on, and you'll be leaving post-tax money behind in your traditional IRA.
(If it sounds like I'm annoyed about this situation from personal experience, it's because I am. :)
By doing the conversion first, you never mix pre- and post-tax money, and your money goes where you want it.
Of course, assuming you eventually do roll over your 401(k) into a Traditional IRA, the Really Annoying Consequence above will still plague you, but at least you'll have cleanly converted that first post-tax amount.