You would have to code the parameter and condition so that it accepts the word ALL.
For example, let's say you are working with ITEM_NUMBER and you have thousands of items. You want the user to either be able to key one or more items or access ALL items.
You need to create a Boolean condition like this:
ITEM_NUMBER IN :ITEM_PARAMETER
UPPER(:ITEM_PARAMETER) = 'ALL'
So if the user keys the word ALL or all or All or any variant of the word that will be determined as true by the condition and all items will be selected. If the user does not key ALL then the other half of the condition will be applied and the Item must be a valid item as contained in the list of all items. You can even create your parameter with a default of ALL