Is it possible to add/update asset via rest api in asset management module of SapphireIMS?

Yes, SapphireIMS supports Rest API for creating/updating/searching asset in SapphireIMS system. For more details, please refer "Asset’ section in left side navigation in our online help link

Please note that SapphireIMS Asset API use key and token based authentication. For more details, please refer ‘SETUP YOUR ENVIRONMENT’ in the above online help link provided.