Catalyzes the transfer of a ribitol 5-phosphate from CDP-L- ribitol to the ribitol 5-phosphate previously attached by FKTN/fukutin to the phosphorylated O-mannosyl trisaccharide (N-acetylgalactosamine- beta-3-N-acetylglucosamine-beta-4-(phosphate-6-)mannose), a carbohydrate structure present in alpha-dystroglycan (DAG1) (PubMed:26923585, PubMed:27194101, PubMed:29477842, PubMed:31949166). This constitutes the second step in the formation of the ribose 5- phosphate tandem repeat which links the phosphorylated O-mannosyl trisaccharide to the ligand binding moiety composed of repeats of 3- xylosyl-alpha-1,3-glucuronic acid-beta-1 (PubMed:25279699, PubMed:26923585, PubMed:27194101, PubMed:29477842, PubMed:31949166). {ECO:0000269|PubMed:25279699, ECO:0000269|PubMed:26923585, ECO:0000269|PubMed:27194101, ECO:0000269|PubMed:29477842, ECO:0000269|PubMed:31949166}. This is the function of FKRP (fukutin related protein, ENSG00000181027).