shopt -s expand_aliases # turn on alias support in noninteractive shell

# define the function
alias_git_as() { alias "git_as_$1=GIT_AUTHOR_NAME=${2@Q} GIT_AUTHOR_EMAIL=${3@Q}"; }

# use the function
alias_git_as "login" "Surname Prename" "user@company.de"

# use the invoked alias
git_as_login

declare -p GIT_AUTHOR_NAME GIT_AUTHOR_EMAIL
alias -p