How to use off board components in Altium and have them show up in net list & BOM?
Create symbols for the off-board parts and define the type as "mechanical" parts. 



You can choose "Standard", "Mechanical", "Graphical", "Net Tie (in BOM)", "Net Tie" or "Standard (No BOM)". 

Show them on the schematic- using text etc. so they stand out if you need to verify or change them. They will then show up in the BOM automatically. 

You can do a similar thing with mounting holes, except they would be "Standard (no BOM)". 



You can also create parts and footprints that have no pins, which means that if you put them on the schematic they will automatically be transferred to the 3D PCB layout. This is useful for parts that you want to appear in the 3D model (and BOM) but have no electrical connections on the PCB.