When you donate paid time off (PTO) to a fellow employee, you don't actually lose money. You get paid the same, either way. You work a little more. It's essentially the same as volunteering labor. As a result, you can't claim a charitable donation. You didn't donate anything with tax implications.
In order to claim a charitable donation, you have to make the donation out of after-tax money or goods presumably purchased with after tax money. I don't see how you could do that with a PTO "donation" to another employee.
See here for example, the leave-sharing examples. Note that if your employer donates money to a charity for you, you can deduct your PTO donation but you have to pay taxes on it first. That hardly seems advantageous to you. It basically gets you back to the leave sharing example, only with more paperwork.