You can use setValue(value)
to append a value into a cell, keeping the original value intact:
If the destination sheet is the active sheet:
var dest = SpreadsheetApp.getActiveSheet().getRange("B3");
var original = dest.getValue();
dest.setValue(original+value1);
If the destination sheet is the named one:
var dest = sheet.getRange("B3");
var original = dest.getValue();
dest.setValue(original+value1);
References:
setValue(value)
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