Does the 457(b) plan allow for the rollover of other retirement funds into it? And do you have very specific reasons for wanting to roll over your SEP-IRA into the 457(b) plan instead of into some other IRA plan with a different custodian? For example, if you already have a Traditional IRA, is there any reason why your SEP-IRA should not be rolled over into the Traditional IRA?
With regard to the question about separate accounts, once upon a time, rolling
over money from an employment retirement plan (e.g. 401k) into a Traditional IRA
required establishing a separate account called a Rollover Traditional IRA so that
the rolled-over money (and the earnings thereon) were not commingled with standard traditional IRA money resulting from personal contributions). This was so that the account owner had the option of rolling over the separately kept money into a new employer's retirement plan (if such a rollover was permitted by the new 401k
plan). If one did not want to ever roll over money into a new employer plan, one had to write a letter to the custodian telling them that commingling was OK;
you never wanted to put that money into another 401k plan. The law changed some time later and the concept of Rollover IRAs holding non-commingled funds has disappeared. With that as prologue, my answer to your question is that perhaps the law did not change with respect to 457(b) plans, and so the money that you want to rollover into the 457(b) plan needs to be kept separate and not commingled with your contributions via payroll deduction to the 457(b) plan (in case you
want to ever roll over the SEP-IRA money into another SEP-IRA).
Hence, separate accounts are
needed: one to hold your SEP-IRA money and one to hold your contributions
via payroll deductions.