Add someone else to employer-sponsored health insurance?

The correct answer is "he needs to verify with his employer if this is possible."

But, does it really make sense that a company would let an employee add anyone who is not a dependant/spouse/child to be added to their plan?

The exact answer to this question depends on the employer's policy and insurance plan's rules, so the only way to answer it is by asking your (brother's) HR department.

Generally speaking, yes, this can be possible. Legally, there's nothing stopping an employer and/or insurer from adding anyone you want to your insurance plan (if you're willing to pay for it).

However, there is one important tax implication: your brother will owe extra income tax on any portion of the insurance premium that is paid for by his employer, and must pay any employee share with post-tax dollars. This differs from self, spouse, and child coverage, where you usually don't have to pay any income tax on health insurance.

This tax implication is why some companies/plans do not allow you to add anyone other than a spouse or child. The rules are complex and they don't want to deal with the paperwork.