To convert package files to Sims3pack files you will need to download the Sims3pack File Generator.
First, copy the package file you want to convert to a working folder on your desktop.
Find a sims3pack file you already have, it doesn't have to be the same as the package file you want to convert. (This one will be used as a base file. Don't use world, lot or sim files for this) and copy it to a work folder on your desktop. Now you should have the package file and a Sims3pack file in your work folder.
Open the Sims3pack Generate Tool and on the left side, check the box “Package file generate new GUID” and select “package to sims3pack (single)” if not already selected.
Note: you can set it for multiple files but I only do them one at a time.
Next, click “Set Sims3pack File Version” to bring up the sims3pack file version box.
Click “Import from Sims3pack” and navigate to your work folder with the Sims3pack file. Select the Sims3pack file and click Open
Add a name for the file and a description, then click “Apply to All”
Move to the empty box below the columns Guild etc…
Right Click and click “Add”
Your work folder should open up. If not, navigate to your work folder and Select the package file then click "Open"
With your package file added, click "Generate Sim3pack"
This will open you work folder. Select the sims3pack file. This will replace the long numbers to the name of the package file.
Click "Save" and you're done.