url rewrite on top of existing site with url rewrite

Hello, just setting up my site, trying to better the permalinks with the url rewrites but I’m having difficulty because my existing website in the root already utilizes url rewrites and adding the WP rules to the .htaccess files messes everything up. I have WP in a /blog/ subdirectory, so I only need these specific rewrite rules to process calls into that directory. What WP tells me I need to add is:

RewriteBase /blog/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blog/index.php [L]

How can I change this to work around my existing rules from the root? I’m not much of an expert here so any help would be greatly appreciated