From a Run Script action, use something like this:
Application.Macros(vbldMacroTemporary).Add "MACRO_NAME", "macro value"
http://www.visualbuild.com/Manual/macrosproperty.htm
http://www.visualbuild.com/Manual/macrosaddmethod.htm
http://www.visualbuild.com/Manual/runscript.htm
If the VBScript code is external to VBP (i.e., a .vbs file), you would need to instantiate the object model and store in a project (loading the appropriate .bld file first) or global macro.
http://www.visualbuild.com/Manual/objectmodel.htm
http://www.visualbuild.com/Manual/applicationobject.htm