Oh, it occurs to me later that you may have made some logical leaps to arrive at a desire to “parameterize” and “customize” a model. These terms traditionally imply more complexity than you may have intended. You may simply want to adjust the size of the model a little for your own needs, which is technically entirely different.
As I mentioned in my previous reply, STL files contain “mesh” models. These can certainly be added to, removed from, stretched, and shrunk without any reference to the term “parameterize” or “variables.”
Assuming I’m on the right track, there are numerous options available for this, such as Meshmixer (no longer supported), Wings3D, Blender, and others. These all have steep learning curves, though.
If your goal is to make a relatively simple size change for personal use, as opposed to making the model configurable by other users in the classical “parameterized” fashion, then you may be able to do so using only BambuSlicer itself.
Within the slicer, you can scale parts or slice them up and make adjustments before “merging” them back together.
For example, if you want to stretch just one part of a model in one direction, you could perform two slices to isolate that piece. Then, you can scale the now-separated object, move the original parts further apart to make room for the longer piece, and once the pieces are aligned where you want them, you can select them all and use “Merge” (previously “Assemble”) to make them into a single object again.
You can save your new model by right-clicking and selecting “Save as single STL” or simply save the project as a normal 3MF.
Similarly, if you want to move two parts of a model closer together without changing the overall scale, you can make a slice, move the pieces closer so that they overlap, and then merge them. Merging will remove the overlap, leaving a single part composed of the overlapping pieces.
When you slice using the slicer software, there is an option to “slice to parts.” Parts are pieces of the same object, even if they’re not actually connected. As such, you cannot scale an individual part without affecting all the other parts of the object. To make the parts into objects that can be individually scaled, use the right-click “Split => To objects” feature.
Hope that helps.
P.S: We speakers of the Queen’s English (as if!) spell parameterize as parametrise. I’ve tried for years to get the folks in the US to spellinate stuffs correctlier but they don’t seem to care for it. 
