If you're talking about Fielder's contingency theory of leadership, you certainly could propose mediators and test them. It would be good to propose a specific mediator for each of the factors that define the contingencies. In the original model, these include Leader-Member Relations, Task Structure, and Positional Power.
Of course you can use both mediating and moderating variables in the contingency models. You can refer to the following articles for example
1. Forms of Contingency Fit in Managerial Accounting Research—A Critical Review,Jonas Gerdin, Jan Greve, Accounting Organizations and Society 29(3):303-326, DOI: 10.1016/S0361-3682(02)00096-X
2. Alternative Forms of Fit in Contingency Theory, Robert Drazin and Andrew H. Van de VenAdministrative Science QuarterlyVol. 30, No. 4 (Dec., 1985), pp. 514-539