Post-insert M3 nuts into extrusions

Hi, how can I insert nuts into the 1515 extrusions after assembly?

The question probably is rather how can I fasten a screw after everything is assembled and no preloaded nuts are left over?

In my case, Door_Latch_x2.STL has to be fastened somehow else, because I did not pre-load the nuts. And after re-verifying the Assembly Manual pdf file for v0.1, if I'm not mistaken, the manual forgot to pre-load them.