Can I Purchase Resource Packages When I Still Have Valid Ones in Use?

Yes. You can buy resource packages on top of the valid ones, and older packages are preferentially used.

Assume that you had 1.2 TB of file data and you purchased a 1 TB resource package for one month on August 15 and another 1 TB package for one month on August 20. Then, from August 20 to September 15, the first 1 TB package was used up, and the second package was used 0.2 TB. If the first package was not renewed, the second package would be used up during the time from September 15 to September 20.