In the report you can create two different datasets one pointing to relational and other to OLAP. And then you can use Lookup/LookupSet function to join data from those two datasets.

OR

In the stored procedure, you can write an MDX query in a variable (say @mdx) to get the OLAP data and save it to a temp table. For that you need to create a Linked server (to OLAP DB). And use this @mdx variable in an OPENQUERY command (alongwith insert and select statements to fill the temp table). Then you can join the temp table along with relation table to get the appropriate data.