Extentions: can be of any length, all contracts can be. I remember Gavin Creel didn't have a 1 year contract for Hair. IF I remember correctly, they were paying pretty close to (or at) scale for all actors. The bargain they reached gave him a shorter contract at a low salary, then he renegotiated (around Tony time, if I remember correctly) for something more fitting his experience.
As to concerts, that's likely something else that can be easily written into his contract, as they are now "pre-existing"
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