if you won´t to add a foreignkey to sf_guard_user put this on top off you schema.yml file ;)

propel:
  _attributes :          { package: "plugins.sfGuardPlugin.lib.model" }
  sf_guard_user:
    _attributes:         { phpName: sfGuardUser }
    id:

now you can simple add a foreignkey

propel:
  tbl_user_profile:
    user_id:             { type: integer, primaryKey: true, foreignTable: sf_guard_user, foreignReference: id, onDelete: cascade }

or

propel:
  tbl_user_profile:
    sf_guard_user_id: