can I define own variables?

Hi,

can I define connection-global variables within a ODBC connection ?

Like that:
connect to the DB
set my_user_id = 42
Later a trigger would set a timestamp and the user-id when a record gets
updated.

obviously different connections would need differnt variable-values.

the variable should only live until the connection gets terminated.

this user-id represent users of my application and I can't use postgres'
internal user-ids because I have the data sitting in a operational
server and another one for developing and testing, so the postgres ids
arent necessarily consistent between the 2 server-systems.
My application has it's own user management and those keys are used for
rel. integrity.