One way to do this is to setup an HRA (Health Reimbursement Arrangement). The most common way to do this is to buy cheap high-deductible insurance plans, then setup a tax-advantaged account with which you can reimburse employees for eligible medical expenses out of (in your case, you could choose to pay out 100% of expenses before they hit the insurance deductible). There are a lot of different ways to do this, but the way to set it up is to contact a third party administrator to do the administration for you (depending on the size of your company it's usually somewhere in the range of $5-$15 / employee / month to manage the administration). I believe there are significant tax advantages to this approach as well, but I'm less familiar with that area.

If you PM me your general location, I can probably give you a few companies to contact in your area who do this sort of arrangement.