Yes, you certainly can. I do. Set up an account with the same broker who has your Roth IRA. Call them to do so, transfer some money into that account and you are in business.
You can invest a total of $5000 into a ROTH and/or a Traditional IRA this year. You can (for retirement investing) only invest now into a 401K sponsored by a company you work for.
But, you can, outside of retirement IRS invest in anything you want. I suggest Mutual Funds (Fidelity, etc.)