You can use a simple AppleScript which creates a new Pages document with the specified template, adds a new page to the document and then saves the document as a template with the specified name. Here it is:
tell application "Pages"
activate
set thisDocument to make new document with properties {document template:template "Template Name"}
tell thisDocument
make new page with properties {template name:"Image with Text"}
end tell
set templatePath to (path to desktop as text) & "Template Name.pages"
save thisDocument in file templatePath as "template"
end tell
Make sure to replace the two "Template Name" 's with the wanted name of the template, replace "Image with Text" with the name of the page template you want to add to your document, and replace "path to desktop as text" to the location where you want to save the template in your desktop.
Note: Saving a document as a template will overwrite any existing template with the same name, so if you want to replace the previous template, you might want to use the same name.
make new page with properties { template:"Page template" }
there is something likemake new page
and thenapply property {template:"Page template"}