It doesn't matter where the money comes from. If you are eligible to contribute to an HSA and have not already contributed the maximum, you can contribute additional amounts up to the maximum annual limit for your type of coverage. The contribution does not have to be made through your employer. If the particular HSA that you have doesn't permit contributions other than those deposited by your employer, you can always open another HSA and contribute to that to make up the difference. All of an individual's HSAs are treated as if they were a single HSA.
