You will need to use dpkg-deb to convert a Cydia DEB file to an IPA file. The Cydia Extender app includes this tool, which allows you to extract the contents of a DEB file and then repackage them as an IPA file.
Step 1: Install Cydia Extender on your jailbroken device.
Step 2: Download the DEB file you wish to convert to your device.
Step 3: Open Cydia Extender and tap on the "Extensions" button.
Step 4: Tap on the "Import DEB" button and select the DEB file you downloaded in step 2.
Step 5: Cydia Extender will now extract the contents of the DEB file.
Step 6: Once the extraction is complete, tap on the "Create IPA" button.
Step 7: Cydia Extender will now repackage the extracted files as an IPA file.
Step 8: The IPA file will be saved to your device's storage. You can now use iTunes to install the IPA file on your device or share it with others.
Note: Please ensure that the jailbreak is still active and that Cydia extender is fully functional before proceeding; otherwise, this method will fail.