FAR 2 defines "Contract" as including orders, thus all clauses and regulations applying to contracts also apply to orders.  FAR 52.232-22 applies to cost remibursement task orders and allows for incremental funding. DFARS 232.703-1 states that a fixed-price contract can only be incrementally funded if it is for severable services, but there are no similar restrictions for cost remibursement. So, in answer to your questions: Yes, you can incrementally fund your cost remibursable task order, and No, it does not matter if there are severable or non severable tasks. 

Best regards, Professor of Contract Management