root floder for sites running on multisite installation

hi,
I recently installed WPMS and I’ve mapped few domains to it using subdomain method.

mymainsite.com is where WPMS sits.

sit1(dot)com and site2(dot)com are mapped using plugin to site1.mymainsite.com and site2.mymainsite.com.

Question is, I want to have a file in the root directory for site1(dot)com, for e.g. welcome.html so that a visitor can call site1(dot)com/welcome.html.

Right now this works when I place the file in the root folder which also hosts WPMS installation. I want to create paths for each domain to achieve the above. So site1(dot)com/welcome.html will work when I place welcome.html file in /site1/ folder.

I’m aware that there’s a blog.dir for uploading files but that’s not what I’m after.

Is this possible.

Thank you in advance.
Maj.