To combine feature permissions from multiple roles into one, follow the steps below.

1) Go to the security role whose features are being exported > Features tab > Export to XML (see first screenshot below).
2) Go to the security role to which you want to add the feature permissions > Features tab > Assign feature permissions > Merge from XML file (see second screenshot) > Select the feature permission XML file(s) that were exported.

Export to XML:
Export to XML function

Merge from XML:
Merge from XML function