Hi, and welcome to the QCAD forum.
‘Complex lines’ or better additional custom Linetypes.
One can define and add these very similar as for example in ACAD.
If you have those Linetypes definitions at hand you can simply include them.
On how to create custom Linetypes definitions there is a lot of information online.
One can source many examples online.
If those include special shapes of a (ACAD) shx shape file then you need to convert the required shape and include it in a cxf font file.
QCAD Pro can edit its own native font files (=line-art) and create new ones.
Also see: Additional Linetypes
Regards,
CVH