Don't feel you have to limit the lich to its given spell list
Access to the spell imprisonment is apparently vital to a lich's continued existence:
A lich must periodically feed souls to its phylactery to sustain the magic preserving its body and consciousness. It does this using the imprisonment spell. [..] A lich that fails or forgets to maintain its body with sacrificed souls begins to physically fall apart, and might eventually become a demilich.
However, you've noticed that, somewhat bizarrely, imprisonent isn't actually in the lich's spell list. What now?
Don't feel the need to limit the lich's access to spells based purely on the spell list provided. This is implied in the MM:
Magic Collectors. Liches collect spells and magic items. In addition to its spell repertoire, a lich has ready access to potions, scrolls, libraries of spellbooks, one or more wands, and perhaps a staff or two.
Liches 'are the remains of great wizards' so (if you feel you need justification for making changes to the template), as DM, you can decide that they prepare their spell list from their 'libraries of spellbooks', just like a wizard PC would.
Finally, I can't tell you why imprisonment wasn't included on the lich's spell list originally, but assuming it was a simple oversight, and either adding it or swapping it in, won't unbalance anything.
After all, with a casting time of 1 minute, it's not a spell that's useful in combat (it would take ten rounds just to cast it), so wouldn't affect CR in any way.