The short answer is that you can’t, although there is a workaround. Essentially, “(Select All)” as implemented in Reporting Services simply does this:
if (all parameter values/members are already selected)
deselect all values/members
select all values/members
You can still get the behavior you want however, assuming that your parameter values are query-based. If you populate your parameter list with a dataset, just use the same dataset to set your default values. This technique will work regardless of whether you’re dealing with SQL or SSAS.
In the attached example RDL the “technique” is demonstrated for both a SQL and SSAS-based data source. (You’ll need the AdventureWorks database and cube samples installed)