Hello M. Florencio,
To insert a "special part" in your 3D model, go to Tool palettes > Advance - Tools > AdvanceSteel special part.
You can also go to the "Listing&checking" toolbar, then press "AdvanceSteel special part" icon.
Pick the location where you want to insert the "special part".
Press OK for the warning message which appears.
In the "Special part" dialog, press "Browse" button and then select the DWG file which contains the "special part" (as a 3D solid).
For your information, if you go to "Behavior" group (on the left of the dialog) you can unselect the options "used for numbering" and "used for Lists" if you do not want to get this special part numbered and visible in BOMs.