Having An &&<All&&> Option In An Integer Parameter

I have cascading parameters in my report, and the vast majority of the fields on which the parameters are
based are varchar fields. In each of the drop-down menus on the report, I would like to have an option
at the top of the list called <All> which allows the user to select all of the possible values.

I have done this successfully for all the varchar fields, but there is one parameter which is based on an
integer. This obviously means it won't accept the value <All>. So, as a temporary workaround I have
created an option called -1, which when selected by the user, selects all the possible values in the
list. However, this is likely to be confusing for users if they don't realise that the -1 option means
"all". Is there any way I can make the -1 actually appear in the list as <All>? How would I go about
this?

The available values for the parameters come from an underlying dataset query.