You can use Pro/Cabling and set the length and then move the parts, or you can use pro/piping, create a flexible pipe, set it's length and then move the parts it is attached to. You can create a curve that references the 2 parts that move and sweep along it but in general the length would not remain constant without some work such as datum analysis feature and relations or similar effort.
Sp can you be more specific about what you want to do and with which package.
I got it figured out. Thanks! I just created an assembly in another. The assembly consists of a cable eyelet attached on one end of the cable and a threaded plug on the other end. I assembled everything using assembly references and it works fine.