If you really DO mean convert a slimline to take a rollerball, then yes, you just about do it, but it's a tight fit. There is no mechanism available for a rollerball - they normally rely on a cap, so its really just a question of making sure the spring at the clip end of your rollerball is long enough, or is packed out to sit in the right place, because a rollerball refill is shorter that a cross style refill.
If actually you mean convert a slimline to take a G2 parker style refill, then the answer is no, because a G2 mechanism requires a wider tube than the 7mm that a slimline gives you.
Having said that, I think you will find that Dan is shortly going to introduce a pretty slim G2 kit, which looks a bit like a streamline. That may fit the bill.
