Hi,
AFAIK,
SAP UI add-on is an installation of SAPUI5 on your SRM Server, which will allow you to use new UI for the shopping cart (PO, GR IV stay in WebDynpro).
Fiori apps for SRM:
Approval of Purchase order, Shopping carts;
Sourcing
My Shopping cart
PO
RFx
etc.
http://help.sap.com/fiori_bs2013/helpdata/en/ed/e1e153ddf60466e10000000a423f68/frameset.htm
Regards
Konstantin