To extend schema.org to support custom properties you usually have to find a more generic type or property that is a superset of your custom term. In the case of playerNumber and playerPosition, I can't think of any property from schema.org that relates to these two custom terms. You could potentially use 'description' even though it is very generic, so something like: property="description/playerNumber" property="description/playerPosition"
In any case, this should not prevent you from publishing your content in RDFa lite. If you are not sure what to do about playerNumber and playerPosition, you can also just leave them out and not add RDFa Lite for them. You don't have to annotate every single item of your page, you can always improve that later.
You could also ask your question on http://lists.w3.org/Archives/Public/public-vocabs/, it is the official mailing list for the scgem
BTW, the rest of your markup looks good. Just make sure to move the typeof="Organization" into the span that has property="memberOf", since you are defining a new entity organization entity, it should be in the same HTML element as the property that creates it. Also, in schema.org, Organization is spelt in US English with a "z".