SUBSTRING and IF statement
Another alternative is to use STUFF function.

IF OBJECT_ID('tempdb..#ExampleValues') IS NOT NULL
    DROP TABLE #ExampleValues

CREATE TABLE #ExampleValues (
    ExampleValue VARCHAR(100), 
    UpdatedValue VARCHAR(100))

INSERT INTO #ExampleValues (ExampleValue)
VALUES
    ('112344353-0'),
    ('11443-8'),
    ('112344-9'),
    ('11234435348'),
    ('11234448712')

UPDATE T SET
    UpdatedValue = CASE 
        WHEN T.ExampleValue NOT LIKE '%-%'  -- If the literal does not contain a hyphen
            THEN STUFF(
                T.ExampleValue,             -- Put in the literal
                LEN(T.ExampleValue),        -- Just before the last character
                0,                          -- While replacing 0 characters ("stuffing" in between)
                '-')                        -- Place a hypen!
        ELSE
            T.ExampleValue END
FROM
    #ExampleValues AS T

SELECT
    T.ExampleValue,
    T.UpdatedValue
FROM
    #ExampleValues AS T


Result:

ExampleValue    UpdatedValue
112344353-0     112344353-0
11443-8         11443-8
112344-9        112344-9
11234435348     1123443534-8
11234448712     1123444871-2