You can, but in my experience you will find clerks try to use excuses to avoid selling them (we don't do advances at this time of day, I can't do it as I'm the only window open, there's too much of a queue come back later, or the like). As a courtesy it might be worth writing out the exact journey details, and ticket type codes if you can.
I did it at New Street at 6:30pm, gave her a list of 4 journeys and also asked for a Daytripper for tomorrow. Gave her 4x RTVs totalling 30.00. Chip and pin failed repeatedly for the 8.50 balance so she put it through again. I think she ignored my reserved seat preferences but not the end of the world.