In some cases you can, and in some cases you can not.
Many producers have access to library material or in-house stock shots that can be repurposed for new shows. Most of legacy material is in standard definition and this poses a question whether such material is suitable for use in a high definition show.
In most cases you can upconvert and use SD stock shots. They will appear significantly softer than native HD material but this negative impact can be drastically reduced through judicious editing.
It is difficult to use SD clips shot on video in 1080 23.98p shows. Unless the stock footage originated as a 24p material such as film, it is difficult to avoid motion artifacts when changing the frame rate from 29.97 to 23.98.
Lastly, your network may have the final word if upconversions from SD are acceptable.