Attaching a frame to an existing bag works but often demolishes the optics. What you can do ist simply strap the bag to a naked frame, using the frame as a carrier. You'll however soon be annoyed by the bulkyness of the frame. There are dedicated front-carriers on the market that are less bulky like i.e. this one from Fahrer-Berlin. There are also ones from H&H that you can i.e. get from Asia via ebay. So you have the choice between:

- getting a dedicated Brompton bag (plenthora of makers and possilities)
- modifying your existing bag with a frame fixed to it
- strapping your existing bag to a frame or a front carrier

Most (but not all) people end up with option one sooner or later as it turns out that the dedicated bags have their advantages in daily life for them. If you want a fixed attachment to your existing bag you are not limited to Brompton frames - there are also adaptors to the carrier block available out there that give you a bit more freedom in how to modify your bag (or you can make one yourself - it is not too difficult).