Yes, it's quite possible to transform arbitrary features from an external system into a format Kaldi can understand. You'll want to do something like this:
1. Look at the format of Kaldi features. For example, if you have MFCCs in data/train/feats.scp, you can run copy-feats scp:data/train/feats.scp ark,t:feats.txt to transform the features into a plain text format that you can read.
2. Once you understand the format the features need to be in, write some script that transforms your DBN features into this plain text format, e.g., save it to your_feats.txt.
3. Now you'll want to transform your features into the ark/scp format that Kaldi typically uses. You can do something like copy-feats ark,t:your_feats.txt ark,scp:your_feats.ark,your_feats.scp to do that.
4. Create some data directory where your new feats will live. E.g., run utils/copy_data_dir.sh data/train data/train_your_feats. Then copy your_feats.scp into data/train_your_feats/feats.scp
5. Now you should be able to use data/train_your_feats to train your UBM or i-vector extractor