"My wife is over 55 years & I understand she can contribute $1000 to her own HSA."
Yes. Eligible individuals age 55 and older may increase their HSA limit by up to $1000. Eligible individual means they are covered by HDHP medical insurance and have no other medical coverage, such as Medicare, FSA or a secondary policy.
"Is the amount "after tax dollars"? In other words can she just write a check for $1000 to her own HSA?"
The extra contribution can be pretax OR after tax money. You could write a personal check or ask an employer to make pretax deposits to either spouse's HSA.
"We file our taxes jointly"
When you both have HSAs and file jointly, your tax filing will include a Form 8889 for each of you. The amount of HSA contribution allowed for each spouse will depend on the type of HDHP insurance coverage for each person. If you have family HDHP coverage then your limit is $6750. This amount can be divided anyway you chose between each account. Then add $1000 to each person age 55 or older. If you each have self HDHP coverage then your limit is $3350 for each account then add $1000 to each person age 55 or older. These limits assume the same HDHP coverage the entire tax year.
