Hi,
If you do not transfer your shares to a pension immediately when the scheme ends, you can still transfer them up to 90 days later. You may have to pay Capital Gains Tax if they go up in value between when you buy them and when you transfer them.
Your ISA provider will need to agree to the transfer of shares. They will count towards your £20000 limit and cannot be in addition to the £20000 limit. You can see guidance here:
Tax and Employee Share Schemes
Thank you.