You can do what you asked by first fetching the custom field value on page load and then set it to some hidden field.On submit you will have that value in backend.

Step 1: Get value from Expando Column table:

Integer webCntMaxSize = 0;
long jrnlClassNameId = ClassNameLocalServiceUtil.getClassNameId(JournalArticle.class.getName());
long company_id = user.getCompanyId();
ExpandoTable jnrnlTable = ExpandoTableLocalServiceUtil.getDefaultTable(company_id, jrnlClassNameId);
ExpandoColumn jrnlColumn = ExpandoColumnLocalServiceUtil.getColumn(jnrnlTable.getTableId(), "mxWebContentSize");
String value = jrnlColumn.getDefaultData();
System.out.println("value in server.jspf:"+value);
if (value != null) {
webCntMaxSize= Integer.parseInt(value);
}

Step 2:Set the value in hidden field as below:

<aui:form method="post" name="fm2">
<input name="fieldName" type="hidden" value="<%=value%>" />
<</aui:form>