Re-reflowing a populated PCB
If you are only doing a few for your development then I would suggest you manually solder the missing parts on, using an iron or a hot air tool.  Put on the flux as you said and then reflow just the added components.

Modules sometimes try to use a slightly higher temperature solder so the standard reflow profile will not remelt them, however as mentioned in the comments components will not just drop off unless they are heavy and on the bottom of a hot board.

When wave soldering surface mount components it is usual to use a glue dot to hold the parts in place but with IR reflow this is usually not done as it would need a dot dispenser in addition to the solder stencil process.

Multiple reflow cycles are not recommended but I would certainly think that taking the occasional risk may be worth it on a development board, worst case is it does not work.