Custom field on wp-signup.php

Hello WordPress Community!

I run a small WP-multisite network and need to put the sites in categories (i.e. Football Blogs, Baseball Blogs, …). I already have setup a settings-page (Options > Blog Categories) where users can choose the right category and now i want to put a selectbox like this on the wp-signup.php:

<select name="my_blog_cat">
	<option value="1">Football Blogs</option>
	<option value="2">Baseball Blogs</option>
</select>

I have searched with google and the forum search, but i can′t find a userfull tutorial. On the settiongs-page i save the category with update_option('my_blog_cat','$my_blog_cat'), but how can i save this option in the registration process and how can i do it with in a plugin, (i don′t want to hardcode it in the wp-signup.php)?

Best regards and sorry for my bad english!