I recently did much the same when I purchased a new iMac.
This post made it so easy for me to get all of my add-ons and themes back into a new installation:
If this link doesn’t work - the reply was by @svsmailus Simon as follows:
"For anyone interested, the path I chose was as follows (after a clean install (in my case El Capitan)):
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Install Rapidweaver and add your license
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Go to your backup (in my case SpuerDuper) to: /User/Username/Library/Containers/com.realmacsoftware.rapidweaver/Data/Library/Application Support/Rapidweaver
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Copy the whole folder to your Desktop or some place else from your backup
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Go into the Rapidweaver folder you just copied and select all the “.rapidweaverplugin” files and right-click and “open with” > Rapidweaver 7. This will open Rapidweaver and install the plugins.
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In the Rapidweaver folder you copied from your backup now go to the “Stacks folder”. Select all the stacks and right-click and “open with” > Rapidweaver 7. This will install all your stacks.
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In the Rapidweaver folder you copied from your backup now go to the “Themes folder”. Select all the Themes and right-click and “open with” > Rapidweaver 7. This will install all your Themes.
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In the Rapidweaver folder you copied from your backup now go to the “Snippets folder”. Select all the Snippets and right-click and “open with” > Rapidweaver 7. This will install all your Snippets.
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In Rapidweaver click on “Library” and “Updates”. This is only necessary if you have not updated your Rapidweaver library in some time. Update all items highlighted.
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You will need your license keys for various stacks/plugin etc.
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Open a project and you should be good to go."