Yes, you can use it to isolate the RNA. But longer the cell pellet is incubated at -80c the quality of RNA will be compromised.

Yes, you can definitely use Qiagen RNAeasy kit to isolate your RNA. However, as Hemant Joshi mentioned that your RNA yield will reduce the longer you store it.

Yes, you can definitely use it but as mentioned by the other dignified Researchers, the quality of RNA will be compromised with longer cryopreservation.

Thank you everyone for your responses. As I have to extract RNA from sorted leukemic cells, I have planned to add RNAlater after pelleting them down. And then extract RNA using the kit at a later date