Sublime Text 3 - Regex find/replace first captured group with second captured group?

Question

I have two XML files I'm editing using Sublime Text 3. The first file acts as a template, while the second file is based on data export that is a batched XML file with multiple references to the targeted tags. I am looking to extract all found references and paste them into the placeholder in my template.

I'm using the Find in files option to find both my replacement text placeholder (i.e. <--Replacement XML-->) in my template, along with the XML that should replace it in the secondary file (...):

((<!--Replacement XML-->)|((?s)<activities>\s*\K(.*?)(?=\s*</activities>)))

The capturing groups work fine to both target both capturing groups, but I'm wondering if it's possible to use the replace feature to replace "<--Replacement XML-->" with the results found using the second capturing group.

I've seen references to identifying the capture group using the sequence of the capturing group (ex: $1 or \1), but it doesn't seem to work and instead tries to replace all references rather than my placeholder.

Sample taken from files:

Template:

        <placeholder>
         <!--Replacement XML-->
        </placeholder>

Bulk Data

        <activities>
         <part>Some String</part>
         <part>Some Other String</part>
        </activities>