.htaccess redirect if file exists in subfolder - apache

Question

I have the following challenge. Assuming the following file is requested from the server:
/pages/subfolder/mypage.php
Is there an option using mod-rewrite to do the following:
Look into a specific folder f. e. /USERMOD if the requested file (and structure) exists
If Yes, execute this file with all parameters
If Not, execute the requested file.
The background would be that the rewrite looks if there is a replacement under /USERMOD having the same folder structure. If an replacement exists use this one otherwise us the original file.
Actually (but doesn't work) I have is:
RewriteCond %{REQUEST_URI} !^/USERMOD/ [NC]
RewriteCond %{DOCUMENT_ROOT}/USERMOD/$1 -f [NC]
RewriteRule ^(.*)$ /USERMOD/$1 [L]
Thanks for any hint and help!
EDIT: The full .htaccess
RewriteEngine On
RewriteCond %{REQUEST_URI} !^/USERMOD/ [NC]
RewriteCond %{DOCUMENT_ROOT}/USERMOD/$1 -f [NC]
RewriteRule ^(.*)$ /USERMOD/$1 [L]
RewriteCond %{REQUEST_FILENAME} ^(.*)\.(php|css|js|gif|jpg|jpeg|png)$ [NC]
RewriteRule ^(.+) - [L]
RewriteCond %{REQUEST_URI} (.*)?/admin/(.*)
RewriteRule ^(.+) - [L]
RewriteCond %{REQUEST_URI} (.*)?/images/(.*)
RewriteRule ^(.+) - [L]
RewriteCond %{REQUEST_URI} (.*)?/templates/(.*)
RewriteRule ^(.+) - [L]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^(.+) - [L]
RewriteCond %{REQUEST_FILENAME} -f
RewriteRule ^(.+) - [L]
RewriteCond %{REQUEST_FILENAME} -l
RewriteRule ^(.+) - [L]
##boosted CONTENT
RewriteRule (.*/)?info/([A-Za-z0-9_-]+)\.html.* shop_content.php?gm_boosted_content=$2&%{QUERY_STRING} [PT,L]
##boosted PRODUCTS
RewriteRule (.*/)?([A-Za-z0-9_-]+)\.html product_info.php?gm_boosted_product=$2&%{QUERY_STRING} [PT,L]
##boosted CATEGORIES
RewriteRule (.*/)?([A-Za-z0-9_-]+)/?.* index.php?gm_boosted_category=$2&%{QUERY_STRING} [L]

Answer 1

Have your initial rules like this:
RewriteEngine On
RewriteRule ^USERMOD/ - [NC,L]
RewriteCond %{DOCUMENT_ROOT}/USERMOD/$1 -f [NC]
RewriteRule ^(.*)$ /USERMOD/$1 [L]
# rest of your rules come here