You're going to have to work around this a little bit yourself. What you will need is a daemon or frequently run job that fires off something like fetchmail which is configured to login to your imap account, suck off your mail and either forward it to your gmail address or manually deliver it into another imap account (again, your gmail box).
This will need to run from somewhere consistently in order for you to have your mail up to date, but it doesn't have to be the box you check your mail from. You will be able to use your gmail from anywhere as usual as long as that fetchmail or similar is running somewhere acting as that little tunnel porting mail from one place to the other.
Best Answer
Are you not able to just have the IMAP account forward to gmail?