Different options to convert brick wall outlet box to junction box with more outlets
This is one of things extension boxes are for

Your usecase is one of several that extension boxes are meant for -- you'll either want to use a standard one-gang metal extension box with THHN in EMT running to surface mounted one gang boxes for the new receptacles, or a surface raceway (sometimes called 'wiremold' after Legrand's brand name for the stuff, not to be confused with a product Legrand calls "cordmate" that is only intended for signal cables) extension box ("starter box") along with matching raceway and surface mount receptacle boxes, with either NM or THHN run in the raceways.

An example of the latter is shown below.