No such function is provided.
You will need to use strel() and then pad the array or crop the array as appropriate.
Note that structuring elements are "slid" over the entire image when in use. You would not be defining a structure element based on data (pixel) coordinates. It would be rather unusual to define a structure element that was an off-center circle (not impossible, but there is not typically any use for such a thing.)
I wonder if you are trying to use strel() to define a circle mask to use once for the entire image, rather than for morphological operations? That kind of processing would typically be done using ROI operations or by mathematical calculations.
