Copy geometry curve to use in assembly

One option: Create a skeleton model in the assembly that defines all of the connection references needed. You can then use the skeleton to create and maintain the connections.

Another option: Create the copy geom in the component needed to set up the connection rather than in the assembly. This will be an external copy geom feature.

https://support.ptc.com/help/creo/creo_pma/usascii/index.html#page/assembly/asm/asm_three_sub/About_External_Copy_Geometry.html 

I think it's not because it's geometry in the assembly, but because it is a sketch. If it's a line, you can define an axis using the two points of the line, then use that axis to assemble things. I tried to use sketch geometry as a reference for assembly and it is not able to be selected.