How to call a stored procedure from a user defined function in SQL Server 2000
Either you need to modify your stored procedure to be a user defined function or the other way around.
One crude way to achieve what you are looking for is to have your exec statement in a batch script and call that batch script from your function. Something like this:
create function
exec master.sys.xp_cmpshell 'C:\storedProc.bat'
....
....
return @return
end
More on xp_cmpshell on MSDN.