That sucks. I'm really glad my employer allows after-tax contributions. I started this job in October 2014, and by then I had already maxed out my pre-tax 401k at the old employer. So if I weren't allowed to contribute after-tax, I would have missed out on the company match for 3 months. That would have cost me $1000.
Frankly it's surprising that any plan wouldn't allow it. My new 401k provider essentially begs you to contribute any money that you can, so they can charge their stupid fees. I can't remember how many reminders I got telling me to roll over my old 401k into my new one, to make it "easier to manage". Um, no thanks. Average fund at new 401k (MassMutual) charges ~0.9% vs. 0.1% for my investments at the old 401k (Vanguard, of course).