I think part of the issue is that you have to be careful with the exact model you have (I found this out!) just saying SGDA is not enough
It looks like the model you have is analog velocity or torque mode only (though even then, the users manual is rather confusing, using part numbers with 2 P options)
If your drives are analog mode only then you need an analog interface card to run them. For LinuxCNC there are about 4 companies that make analog interfaces:
General Mechatronics, Mesa Electronics, Pico Systems and Vital Systems. For Mach there is CS-Lab (CSMIO) and probably many others.
If this is all too overwhelming, you may be better off just buying long shaft step motors or using a shaft extension, if step motor performance is adequate,
since setting up a analog servo system with the required encoder feedback and tuning may be more of a project that you wish to undertake just to re-use
the drives you have.