Preserve key echo among command invocation

[Frank Fischer]

Hi,

if a user starts typing a key sequence and stops for a few seconds
before the sequence is complete, the keys types so far are shown in the
echo area. As soon as the sequence is complete, the area is cleared and
for the next command only the new keys are shown.

My questions is if it is possible to preserve the sequence of keys to be
shown even if a certain command is executed. So I want to write a
command that, when called, does not cause the echo area to be cleared so
that the keys of the following command are appended.

Some commands like `digit-argument' behave like this (typing a number
calls that `digit-argument' put the echo area is preserved until the
actual command is executed). But that's a special case.

Is there way to do this for my own commands?

Thanks in advance.

Frank