As far as I know, there is no explicit way to merge units in the game. If you want to create an unit with a completely freeform troop composition, you can transfer troops from an existing unit (including your SHQ) into a Battlegroup with the 'Transfer/BG' button in the right-side panel, though this comes with some minor penalties, and the last few subunits in a regular unit can't be transferred out. To alter the composition of your standardised formations, you have to discover & operationalize new ones using the Staff Council (and the upgrade existing ones under Unit Admin in the right-side panel). However, you can attach up to 2 independent units to each standardised formation's OHQ under Unit Admin -> Change HQ.
Last time I tried to integrate GR troops into one of my regular units, it appeared they were not compatible with the regular unit's table of organization and equipment, despite being the right general unit type. However, you can transfer them into other GR units and battlegroups.