Answering the OP here...
Yes, according to canon the IN and IISS do maintain bases outside the borders of the 3-I.
Some systems have an out-and-out Imperial facility [a Naval Base or Scout base] in system. For example, Nebeth/Egyrn in the Trojan Reach.
If a world's Allegiance is listed as 'Cs', it is a client state of the Imperium and has agreements to allow Imperial forces vessels basing rights at their facilities. You can find several examples of these in The Beyond and Vanguard Reaches.
Lastly, some multi-world governments have treaties with various clauses that allow Imperial ships passage, repair, or full basing rights.