It is possible to override any of the Send method parameters by using setAttribute DML functions or by writing a custom Message Flow Component (MFC in Message Builder or Java). From the Integration-Process, you then call the MFC in a Sequential Activity that you situate before the Send Activity in the processing sequence.
You can find an example of code that you can use to compile an MBC that overrides the attributes of a Gateway Send method in the file XFBOveride.s4.
The following DML function provides an example of how to override the destination DEAID of AS1PartnerOut:
setAttribute("XFBTPMSendMethodOverride" , "DestinationDEAId" , "AS1PartnerOut");
setAttribute("XFBTPMSendMethodOverride" , "Use_DestinationDEAId" , 1)
For examples of overriding JMS message attributes, refer to the topic, Handling JMS attributes in Integrator.