You can document such an agreement in any way you like. Typically, you might see provisions regarding contingent employment & incentives in your consulting agreement. For an agreement like this to be valuable to you, you would want to set forth the terms in detail, including a good faith effort to obtain Series B, the terms on which the executive position/equity is offered (i.e. as a ROFR, automatic trigger upon financing, performance-based or other metrics, etc.), what exact position/responsibilities would be offered, what type and how much equity would be offered and on what vesting schedule (or, if preferred, you can leave these terms vague). I think it would be unlikely that a startup would want to get that specific in advance. However, it can be tough to put agreements of this sort in any sort of binding writing because the needs of a startup can change over time depending on performance, vision, investors, etc. It's a double edged sword: If you don't put these agreements in writing, then they are not binding, but if they are in writing, they may end up being out of line with actual need of the company (or you) when that time comes along.

So, don't be surprised if a startup doesn't want to be bound to anything in writing and make sure that your agreement for consulting services is fair without assuming you'll get future employment or equity.