And to be honest, it would be best to LEAVE them as seperate accounts.
It will keep your PST file from becoming TOO LARGE, and becmoing corrupt.
Also, it doesnt matter if the account gets cancelled, your outllok will not delete or remove or do anythign with your account or your emails. They will remian just as they are now...no changes. The only thing that will change is that you wont be able to SEND or RECEIVE email on that account anymore......THATS ALL!
My recommendation here is to leave the accounts seperate.
Nothign will be lost, and nothign gets accidently deleted, or mergerd improperly.
I know this isnt what you had in mind.....but its the best way to keep all the accounts contents intact.
Outlook does NOT like to have emails of one account being saved in another account. Outlook flags, and causes you to have to make changes to the outlook App to allow it from constantly warnign you.