it could work also for domain access module in som way?

Have you tested it?

I'm using domain access module and robos.txt module is working only on main site, as normally, on one site, instead I think that with domain access module it just need to put the robots.txt on the folder using the sam name of base url of domain, fo example one my secondary domain is www.hotelsupplies.us so it will need to put a robots.txt (that could be different from robots.txt of main domain) into folder: sites/all/www.hotelsupplis.us/ instead of root as per main domain.
I think it will need to work in this way fordomain access at a quick view, but if you consider to upgrade it, I can o and check it and give you more details about it.

Place the robotstxt module in sites/all/modules to make it available to all sites in a multi-site installation.

But in this way with multisite, peole will be obliged to have the same robots.txt fo al sites, and in some cases it could be a limit?
Anyway this is a super nice tip and if people want to place it different per domain, you can place it into
If placed on root of main domain, using domain accss then robot.txt is the same on all other subdomains, instead putting it into
maindomain/sites/all/themes/themename or also into maindomain/sites/default/domain_name/ notwork, is not working also putting robots.txt into sites/default/files/domain-N (N is idnumber of subdomain) so I tlook that with domain access module actually is impossible to put a different robots.txt per web site, but it could be useful?
I have added details about it here:
http://drupal.org/node/624382#comment-2250092

You are still able to configure robots.txt that is saved in variables table.

I'm not a developer I don' know how to do that, it wil be done from you or from domain access developer or both?

There is nothing to do in robots.txt