It is necessary to go across the outer membrane barrier in order to load cargo into exosomes. During the synthesis process, cargos can be loaded onto synthetic carriers like liposomes. Similar to this, exogenous medications can be added to parental cells to pre-load exosomes in situ. Through planned protein-protein interactions, co-expressed proteins in parent cells can also be directed into exosomes.
For more: pre-loading strategy for exosome