There are severall types of Jetstream refills. Some do not have the black cap on the end like the refill I show below, and for those all you need to do is trim the refill to the same length as the G2 and you are good to go.
The goal with any refill modification is to allow the mechanism move like the Pilot G2 does. The Jetstream works a little differently as the point where the spring rests on the G2 is a bit different from the Jetstream. If you look at it up close the Jetstream front is a little longer, so some extra from the back has to be trimmed off. Here is that close up of the fronts:
Front Closeup of UniBall JetStream on top, Pilot G2 on bottom. Note they are lined up where the spring will rest on the front is in the same place.
UniBall JetStream on top, Pilot G2 on bottom. Note they are lined up where the spring will rest on the front is in the same place.
So from here you need to take off the end piece on the Jetstream:
Next, trim off about 1/4" of an inch with sharp knife. I didn't get a measurement of before, but always trim less because you can always trim off more later and it is nearly impossible to add on more. From here you put back in the end piece. You'll have some of the goo at the end come out and just wipe that off.
Now the refills should be approximately the same length if you line them back up. Insert the modified refill in the pen with the spring in the front and screw it together and you are done.