Yes, you can charge a dSLR from a portable solar system. But, no, you cannot do it from a USB system (without being very slow & inefficient about it). Electricity is much like water... going from 5V USB up to 7.4V is like pushing water up hill, and that requires a pump (complexity you don't need). Plus, these little revolve batteries will not be generating anywhere near enough power to support a dSLR battery under normal use.
The solution is to match a 12V solar panel to the demand for power your camera have. For example, most dSLR cameras have a battery capacity (in terms of recharging it) of approx 16 Watt-hours. If you told me that you would drain the camera battery in 2 days under normal use, I would tell you to top it up every day and the power needed would be 8 Watt-hours each day (ie half).
To offset a demand of 8 Watt-hours per day, I would suggest that the smallest of the 12V panels would do the trick. 4 Watts would be enough, but most manufacturers start at a 5W size...
To compare, the Revolve xeMilo only produces 1.2 Watts.
If you have any other devices to charge, then 5 Watts will come in handy.
Next, you need to store the solar power in a battery pack that can support a charger for your camera battery. Something small like the Novuscell 24 is a good match for a 5W panel...
Lastly, choose a 12V DC charger for your camera battery. DO NOT attempt to use the charger that came with your camera, as this is an AC model designed for home use... seek a DC model that can be used from your car or from a 12V solar kit. I strongly recommend a universal model like the Ansmann Vario as it can adapt to just about any camera model you will have now or in the future, plus will handle the smaller camera & phone batteries, GPS batteries, AA & AAA batteries, etc...
That's about it. A system to properly support your dSLR camera.
Now, having said all that, I have a small universal charger that will in fact charge 7.4V batteries from a 5V USB source. It is slow, but it works...
Note that small solar charger/battery packs like the Revolve only have the solar power to charge approx 20% of your camera battery on an ongoing daily basis (summertime with good sky exposure all day). The storage capacity of the xeMilo Revolve can support one full charge of your camera, but then it is dead, and will take approx 5 days to fill again (you can use it daily of course, but you can only take out what the sun has put in).
I hope this helps!
Personal experience in consulting on portable solar power systems for over 10 years.
Physics degree, plus 15 years in the solar industry.