I think there is no way to actually import a Fusion file into a base feature which I found very surprising.
Once could export convert to DXF and the use the insert command. But this is not really a workflow you want to ask users to use.
However this again is an old issue of Fusion not providing adequate import abilities - STEP import or such should work via the insert command which is common
when like in this case it is even impossible for Fusion to insert a design from the dashboard into a base feature.