Is there anyway to convert an email address into an Appian user type value?

If there is not a plugin available, then you will likely have to write an expression rule to get the information. If you get all users from the specific group of business users, then compare their e-mail addresses to the given e-mail, you should be able to match up the username. The limitations here are that if the email address is used for more than one user then all users will be returned. And if the e-mail address is not used then you will be left with a null value, both of which could cause issues in the process.

Your expression rule would look something like this:
with(
local!allUsers: getdistinctusers(togroup(26)),
index(
local!allUsers,
wherecontains(
ri!email,
a!forEach(local!allUsers, user(fv!item, "email"))
)
)
)

You can use this as well and will work in <17.2
with(
local!listOfUsers: getdistinctusers(
togroup(
cons!YOUR_BUSSINESS_GROUP
)
),
index(
local!listOfUsers,
wherecontains(
"CHANDRASEKHAR.GATTU@GMAIL.COM",
apply(
user(
_,
"email"
),
local!listOfUsers
)
)
)

)

You can use this as well and will work in <17.2
with(
local!listOfUsers: getdistinctusers(
togroup(
cons!YOUR_BUSSINESS_GROUP
)
),
index(
local!listOfUsers,
wherecontains(
"CHANDRASEKHAR.GATTU@GMAIL.COM",
apply(
user(
_,
"email"
),
local!listOfUsers
)
)
)

)