You have gotten some good information from Ottoman and MG, jimmyz. I don't have much to add, but in regard to your specific question, Is it possible to get inside push in a 3-4 defense without blitzing inside backers? , I would have to say that it would be more difficult.
Irrespective of whether the base alignment is a 3-4 or a 4-3, you normally will have four pass pass rushers. In the 4-3, they are the four defensive linemen. In the 3-4 defense, the three defensive linemen are usually joined by a linebacker, most often the OLB on the weak- or blind side. On passing downs, he often is up on the line of scrimmage outside the defensive end. (He's not considered to be blitzer unless another LB also rushes because a blitz by definition requires at least five rushers.)
It does sort of blur the distinction between the 3-4 and 4-3 because the responsibilities of the three remaining linebackers really aren't much different than they would be in a normal 4-3 set. If you are trying to get pressure up the middle, then at least one of the inside LBs almost certainly would have to join the pass rush. If he does, there really is no particular reason why the usually pass rushing OLB wouldn't continue to do so. Thus, you would have a blitz.
The reason behind what I''ve just said is because the DEs and NT playing in a base 3-4 generally are the larger, run defending types, and as such are not usually ideal pass rushers. In a 3-4 alignment, the Linebackers are the principal pass rushers thus it is often more difficult to generate good pressure with only four rushers unless at least one of the Dlinemen is an exceptionally versatile player.