Equivalence from multi-tape to single-tape implies limited write space?

Suppose I have the following subroutine, to a more complex program, that uses spaces to the right of the tape:

$ A$ : “adds a $ at the beginning of the tape.”

So we have: $ $ \begin{array}{lc} \text{Before }: & 0101\sqcup\sqcup\sqcup\ldots\ \text{After }A: & $ 0101\sqcup\sqcup\ldots \end{array} $ $

And it is running on a multi-tape turing machine. The equivalence theorem from Sipser book proposes we can describe any multi-tape TM with a single-tape applying the following “algorithm”, append a $ \#$ at every tape and then concat every tape in a single tape, and then put a dot to simulate the header of the previous machine, etc, etc.

With $ a$ and $ b$ being the content of other tapes, we have: $ $ a\#\dot{0}101\#b $ $ If we want to apply $ A$ or another subroutine that uses more space, the “algorithm” described in Sipser is not enough, I can intuitively shift $ \#b$ to the right, but how to describe it more formally for any other subroutine that uses more space in the tape? I can’t figure out a more general “algorithm” to apply in this cases.