They may cover you, but there will be big exclusions.

They will definitely exclude diabetes itself, and all degenerative diseases
associated with it - vascular (cardiovascular, hypertension, cerebrovascular
- strokes, peripheral vascular system), optical (eyes - retinopathy, glaucoma, cataracts),
renal (kidneys, renal hypertension).

(Bear in mind that diabetes compromises the entire body.)

In effect, a bit chunk of your health will not be covered. Once I told a
friend about Tokyomarine's Medic Plus., a top-up (excess RM 10,000) policy.
They sent a Bengali senior underwriting staff to assess him, found out his
HbA1c was high, and offered him a policy with so many exclusions that it
was virtually useless.

Needless to say, he never bought it, and neither would I.