If you are getting the message like “SAP object XXX cannot be assigned to package XXX” then it can be a good reason to check this post , i hope it helps you to eliminate the situation..
I would like to share an example with you..
Configuring SMW0 for this example ;
Creating object with object name “COMPANY.GIF”
Import the object and try to SAVE
After this , you should get the following message
Click the help button to see details
Go back and now try with creating the object with object name ZXXX
Import the related file again
Click the SAVE button
Now it should work ,you can import the object starting with “Z” to package started with “Z”