Actually you CAN import an old PST into a POP account...it's not importing it to the server, it's just importing it into the local "new" PST. Or you can force Outlook to use the old PST instead of importing the old PST into a new virgin (blank) PST. Either way..it's just a local database. Either use it..or import data from it.
The problem is...from now, moving forward...it's difficult to get 2x computers to "sync" inboxes using POP. Yes...there are crappy ways to almost kinda get close to doing that..checkboxes for "leave mail on server for X days/weeks" before deleting...but....it's a crappy system.. POP is a prehistorci e-mail protocol designed back in the days when people had 1x computer/device checking e-mail. It wasn't designed for multiple devices. IMAP became popular later because it works with multiple devices....sorta mirroring mailboxes because it just creates local "copies". POP downloads the mail...like walking to the end of your driveway and reaching into your mailbox and bringing the mail back into your house. Someone follows you..and checks the mailbox..it's empty!
For businesses...and multiple devices looking identical with e-mail..use a proper biz grade mail system...Exchange. O365.