Yes, you can split a pension with a deceased taxpayer in the year of death.

Turbo tax and many other software will not allow the returns to be prepared together because one spouse's marital status will be 'married' (At time of death) and the other spouse will be widow (Dec 31) 

You will have to manually calculate the best split. If the other spouse does not have pension income that qualifies for the pension income amount, the minimum recommended split is $2000 to maximize the pension income amount.

Where or how can I declare the pension split on deceased person’s return? Can I manually request the appropriate schedule?

Pension splitting can be performed in the year of death. It’s a pro-rated calculation based upon the date of death.

https://turbotax.community.intuit.ca/community/pension-retirement/help/how-do-i-split-my-pension-inc...

You can click on "find" and type in split pension.