As you have stated in your question, you don't want to use a new mapping for this. (I'm also more in favour of "vanilla" vim key-bindings if possible, so I can use my muscle-memory on other systems aswell)
The following vim keys let you paste with an additional space
, without the need to leave insert mode for pasting:
a) Paste after a word somewhere in the middle of the line
- go to end of word with
e
- press
a
to go into insert mode directly after word
- press
space
to insert space after last character
- press
CTRL-r
(cursor turns into "
)
- press
"
for the last copy register (or 0
for the second-to-last, or +
for system clipboard register,...) (*)
b) Paste at the end of current line
- press
A
to go directly into insert mode at the end of line
- press
space
- press
CTRL-r
(cursor turns into "
)
- select preferred register (*)
(*) To see all the copy registers use the command :registers
Also, :help i_CTRL-R
might be worth a read.