Sure. Why would it be a bad thing?
Things to consider though;
- I assume since you started your own agency that you're comfortable writing content that will help a site perform in search. I imagine any decent SEO agency will want some evidence of this. I'm guessing you have samples of different types of content. If not, put some together or get some work under your belt before trying to approach agencies.
- Are your rates reasonable for the companies you're looking to work with? Get a good idea of the price you're worth. If you want the work then you don't want to go in with too high a price, but equally you want to be taken seriously and don't want to undervalue yourself so don't be too cheap.
- Who are these agencies using already? Are they using in-house writers? If so, are they likely to need to outsource? Are they already outsourcing somewhere else and, if so, what can you offer that their current provider cannot?
These aren't meant to put you off, I don't see anything wrong with the idea, just think through the concept and go in with a professional approach. Show them what you can bring to the table so you don't miss your chance by being underprepared. Think through the questions that the agencies you approach might ask and have answers ready for them. If you're serious about this and you are able to do so, try and find local agencies first and arrange a face-to-face meeting and go in with a portfolio of work.