Really depends on the impurity and the material.

Some increase or decrease the critical temperature. Some affect the critical magnetic field (see Ta or Ti in Nb3Sn) by changing the normal state resistance. You can also increase the amount of current that can pass through the material by increasing the number of microstructural features -- smaller grains, say, or bits of non-superconducting material.

Source: working on a degree in superconducting wires