Can H1B extension beyond 6 years be filed using previous I140 approval with no perm labor pending with current employer?

Whether you will be able to use the previous employers I-140 to obtain another three years extension of H-1B visa status will probably depend upon whether the ex-employer has requested withdrawal or cancellation of the old I-140 approval. If that has happened and USCIS is able to connect the dots, the H-1B extension request would be without foundation. If possible, you may wish to check with the former employer to see whether it took any action on the I-140 approval. Due to the limitations of the lawyers.com forums, Alan Lee, Esq.’s (the firm) participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting the question. No attorney/client relationship is or shall be construed to be created hereby. The information provided herein by the firm is general and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice, nor as the basis for making any decisions of legal consequence.

You can only extend your H1B status beyond the six years with if, as you note, your employer has an approved I-140 petition, but also only if the reason you haven't yet been able to adjust status is because a visa number is not yet available.  While you are waiting for a current “Priority Date” in order to file the final Adjustment of Status to Permanent Resident (“Green Card”), your employer will be authorized to extend the H-1B in three year increments until the Permanent resident petition reaches final adjudication.   Your current employer cannot use an approved I-140 filed by a different employer for this purpose if only because an approved I-140 is employer specific.