- Redesign of their pistols to make optimal use of a polymer frame or make entirely new designs.
- Purchase of new equipment or outsourcing the production of the frames to some other company.
- Usage of more expensive alloys or steel for the sub-frame/rails. (It's unlikely that they could make a sub-frame out of ZAMAK without compromising the structural integrity of the frame itself or otherwise making the frame very thick to accommodate the subframe.)
Keep in mind that all of the above require a substantial financial investment as well as a time investment. All of that coming from companies who produce cheaply made firearms offered at bargain prices, aimed at a demographic who is unwilling or unable to spend more money on higher quality firearms, who has everything to lose and little to gain by making the necessary investments to begin production of a new line of firearms with polymer frames.
Yes, one could replace ZAMAK pistol frames with Polymer and end up with a better finished product, but the companies who primarily use ZAMAK aren't exactly known for pumping revenue into R&D, much less for attempting to improve their product lines, so it's a moot point.
In addition, not all firearms designs can be easily altered to replace a metal frame with a polymer frame. If it were that easy and that inexpensive to do, then obviously more companies would have done so by now, and we'd have a wide variety of classic pistol designed which have polymer frames in place of metal.
Lastly, believe it or not, there are certain parts which can be made from ZAMAK which cannot be made from Polymer, such as the receivers on .22 rifles.