Lets assume this was possible in MRP:
The ask from SAP is to switch to this 60 day lead time when there is a stockout situation with the regular lead time.
Now - how would SAP know that the supplier, for this particular instance, would be able to provide you product in 60 days? Did you manage to grease his palms enough? Does he have capacity? It may not be the right thing to use 60 days as lead time since the supplier may or may not be able to meet the deadline.
SAP has to take the lead time into account since its the minimum time taken by the supplier to replenish your stock - any lesser time is subject to negotiation between customer and supplier and supplier's capacity. SAP MRP has no visibility to that.
It is the planner's job to take note that with current lead times and demand, there will be a stockout and ask procurement to negotiate with suppliers.
SAP provides exception messages when exceptions happen with MRP. Planners need to note and understand some of these message codes for complete view of planning situation
MRP Exception Messages list - introductory information
What is rescheduling and how does it work - ERP Manufacturing (PP) - SCN Wiki