The answer here is a definite "maybe." We do not fully understand the processes leading to atmospheric loss on Mars, which are current topics of active research (for example is the erosion steady and slow or fast and violent?). However, most suspected losses are thought to involve the lack of a substantial magnetic field (which is what protects Earth's atmosphere from similar losses). On the other hand, Venus also lacks a substantial magnetic field, yet maintains an extremely thick atmosphere, so perhaps there is some way to work around Mars' small magnetic field.
Finally, there is a question of time - how long do you want Mars' atmosphere to stick around for?
One estimate of the current loss rate is only a few hundred tons per year. That's peanuts compared to what we would need to bring in to terraform Mars in the first place. Which is a another way of saying that there we have much bigger problems involved in terraforming Mars than ongoing atmospheric losses.
