Due to the design of having Thigh armor go over the leg then the lower leg just swapping out at the shin instead of changing the whole knee and lower leg parts (Along with having no peg holes). Is there anything that can be done with Altines' and Altrene's leg armor in regards to adding it to other armors to make a combined mode?

Depending on whether the custom setup you're going for can brook a bit of actual shinki bits in there, you could actually use the leg itself. I'm fairly sure there's at least a couple sets that provide leg-peg- and leg-hole-compatible adapters that are sized for standard armour parts on the other side. IIRC, Zelnogrard's equipment has a couple of those, but I could be wrong - I'm sure she has them for the arms, but she might not for the legs.

If I have time later I'll go dig through my collection a little and see who has them.

As an alternative, while it may not be a 100% secure fit depending on which parts you use, you could thread a longer peg through the hole in the leg armour intended for the thigh, and then 'sandwich' the plate between the part with that peg and another part the peg is plugged into. I've had some success with that, although be very careful when you press things together so you don't stress the leg armour by bending it! This will only really work well with parts that have their pegs projecting from something fairly narrow, of course.

Hope that gives you some ideas!