Read a date filter as input in WEBI report in SAP BO

Your requirement is one of its kinds but can be achieved. What you can try out is first create a brand new object in the context. 

Let's say you provided the date as first input and 5 as a weekly slice in the second input. 

[Date+5]=DATEADD(@prompt('Slice','X'{'Day','Month','Year'},mono,constrained,persistent),5,@prompt('Date','date',mono,free,persistent))

Here X refers to your slice and date refers to the date input.

Then you can just add another filter to the report using this.