convert frame based data to sample based data

hello, I'm doing a demodulator in Simulink based ont RTL-SDR. RTL-SDR could only supports frame mode, each frame may contain 256 or 512 samples. In the demodulator, after synchonization, the demodulator should add a delay on input data, such as 60 samples. Which means, I need a 'new' frame, which contains part of RTL-SDR previous frame, and part of RTL-SDR next frame. But this cannnot be achieved in RTL-SDR block because it does not support that. I tried to use frame conversion block, but apparently that block converts my 256-sample frame into a 256-channel sample. What I need is a one-channel, 256 samples, serial data. Is there any block I can use to achieve this? Thanks a lot!