Thank you Mike!
It works!, although my assets aren't loaded (maybe because my PW installation is under a /test/ directory). I added my rules just before:
RewriteRule /providers/provider1 /test/region-provider/region1/providerName1 [R=301,L]
# -----------------------------------------------------------------------------------------------
# Pass control to ProcessWire if all the above directives allow us to this point.
# For regular VirtualHosts (most installs)
# -----------------------------------------------------------------------------------------------
RewriteRule ^(.*)$ index.php?it=$1 [L,QSA]
# -----------------------------------------------------------------------------------------------
# 500 NOTE: If using VirtualDocumentRoot: comment out the one above and use this one instead.
# -----------------------------------------------------------------------------------------------
# RewriteRule ^(.*)$ /index.php?it=$1 [L,QSA]
</IfModule>
providers/provider1 does not exist in the ProcessWire page tree, it comes from the old CMS.
Finally I tried http://modules.processwire.com/modules/process-redirects/ which works well, but I wanted to learn how to redirect with htaccess, I think is faster.