In my opinion, you would be much better off with an independent milling machine. They are far more capable for almost all jobs compared to an add-on.
I know of no add-on milling attachment for the ubiquitous 7 or 9 X lathes. However, Emco made them in various sizes for their lathes. The Compact 5 & Compact 8 used the same small milling attachment. The Emcomats and Maximats had larger sizes. As you may know, most of the 9 X Asian lathes are a clone of the Emco Compact 8 and I would think one of those would work for you. The milling attachment simply bolts onto the back of the lathe, which had a slightly reinforced pad to accept it. The fixture attached with 4 bolts and simply accepted the post of the milling attachment. The mill is functional, uses an ER-25 collet to hold cutters and is capable of small jobs that don't require a lot of stock removal. If your needs are very simple, this might work.
I will say again that you would be better served with an independent milling machine. Even a small mini-mill will be far, far more capable than one of these attached devices.