Is it possible to keep current implants in and have a breast reduction?

There are a few factors to consider with respect to your question. It is technically possible to do a breast lift or breast reduction when a patient has implants in place.
One thing to consider is the length of time your current implants have been in place. Implants are not meant to be lifelong devices, so assuming you are a young, healthy individual, you will have to do something with the current implants at some point in your lifetime (e.g. replace them, remove them). If your implants have been in place for more than a few years, it might be worth considering exchanging them for new implants (which can be a different size that what you currently have). That could potentially save you from another operation down the line; or at least extended the period for when another surgery is needed.
Another consideration would be your personal history with respect to previous pregnancy, as well as future plans for pregnancy and breastfeeding. Obviously pregnancy and breastfeeding result in changes to the breast, so if you are planning on these events happening in the future, you might consider deferring a breast lift/reduction until after those events have concluded.
The other thing I would want to know for a patient presenting with this question is the location of the current implants. If your implants are on top of the muscle, a lift/reduction may be possible but it would be more difficult in that situation than if the implants are underneath the muscle.