The truth is... you can't. The next best thing is to get the missile addon from Gbombs 5 and stick it in the empty silo. however there are no missiles that look like the one in the other silo. better than nothing. link: https://steamcommunity.com/sharedfiles/filedetails/?edit=true&id=951823689

This gbombs Addon doesn't have a back door.
I guess some can make mod for hbombs that add the hl2 ep 2 rocket.