I should have read this reply first before posting my answer. ;-)
It is also possible to provide study data with information on SEs or confidence limits, i.e.,
metagen(mySMD, myseSMD, lower = lowerSMD, upper = upperSMD, sm = "SMD")
The standard error is only calculated from the lower and upper limits if the standard error of a study is missing.
Best,
Guido