The GeometryReplacer transformer will do this for you.
Set the geometry encoding parameter to: OGC Well Known Text (wkt)
From the documentation:
The OGC variants are useful if interaction with other OGC supporting
systems is required. However, some characteristics of geometries may
have been lost in these modes; for instance, any path will be
flattened into a single linestring. For OGC Well Known Text and OGC
Well Known Binary, if measures are specified in the source, they will
be saved as the "default" (unnamed) measure on the generated geometry.
When converting from WKT or WKB, if the specified attribute has a
blank value, the feature's geometry will be left untouched and a
warning will be output. This is important to remember if a
GeometryExtractor was used to generate an attribute which was used in
this transformer, because that transformer produces an empty value
whenever it encounters a feature with no geometry.
It's also important to note the OGC WKT Precision parameter.
If Geometry Encoding is OGC Well Known Text, this parameter can be
used to control the precision used when converting numerical strings
to floating point.